This invention generally relates to a noise eliminating circuit for an AM receiver, and more particularly to an improvement in a noise eliminating circuit of the type wherein a noise pulse component is detected from an input amplitude modulated wave and in response to the detected noise signal, a main signal path is opened for eliminating said noise component contained in the detected signal on said main signal path.
In a conventional noise eliminating system which is adapted to eliminate a noise pulse component by switching a main signal path for opening it, no problem is caused as far as relatively large noise pulse component appears in an audio signal, but if the appearing noise component is relatively small, there is caused a so-called reversing phenomenon, i.e., a phenomenon where the noise sounds louder than before switching of the main signal path for elimination of the noise component and a great distorsion is developed in the audio signal. It is therefore desired to effect said noise-eliminating switching in response preferentially to a noise component of such a high level that it causes no reversing phenomenon. However in the case where an audio signal exists, a relatively small noise component is not a serious matter to worry about as long as the reversing phenomenon can be avoided. If no audio signal exists, even such a relatively small noise component sounds too distinctively to be disregarded because it is not masked by the audio signal. As a consequence, it is difficult to prevent the phenomenon to a satisfactory extent. Conventional noise eliminating circuits have another disadvantage that in case a noise pulse enters the circuit successively in a relatively short period, the noise detecting function, and accordingly the operation of a pulse generator circuit are disabled, preventing the opening operation of the main signal path for the noise elimination.